I think the description can be improved to make it a bit more clear what is expected.
Original: Create a method one? that accepts a list and a block, and returns true only if the block returns true exactly one (and only one) item in the array.
Modified: Create a method one? that accepts two params, a list and a block, and returns true only if the block returns true for exactly one item in the array.
As a native English speaker, this change makes it clearer, and doesn't require the extra '(and only one)" qualifier.
I think the description can be improved to make it a bit more clear what is expected.
Original: Create a method one? that accepts a list and a block, and returns true only if the block returns true exactly one (and only one) item in the array.
Modified: Create a method one? that accepts two params, a list and a block, and returns true only if the block returns true for exactly one item in the array.
As a native English speaker, this change makes it clearer, and doesn't require the extra '(and only one)" qualifier.